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Mendelian Genetics

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Mendelian Genetics Powered By Docstoc
					Heredity and Evolution
Gregor Mendel (1822-1884)
              Gregor Mendel established
               the basic principles of
               heredity.
              He crossed different
               strains of purebred plants
               and studied their progeny.
              His work illustrates the
               basic rules of inheritance.
The traits Mendel studied in peas
                      Mendel’s work
 Mendel illustrated that inheritance involved individual factors,
   what we now call genes
Monohybrid cross: genetic cross looking at one character or trait
Dihybrid cross: genetic cross looking at two characters or traits
P       Parental generation
F1      First filial generation
F2      Second filial generation
Hybrids
 Offspring of parents who differ from one another with regard to
   certain traits or certain aspects of genetic makeup
                Monohybrid cross
 Results of crosses
  when only one trait at a
  time was considered
             Terms used in genetics
Gene
 A segment of the DNA that directs the production of
  a specific protein, part of a protein, or any functional
  product
 Unit of heredity
 The location of a gene on a chromosome is its
  locus (pl. loci)
Alleles
 The alternate forms of a gene
 Occur at the same locus on homologous
  chromosomes and thus govern the same trait
Alleles
              Terms used in genetics
Heterozygous
 Having different alleles at the same locus on members of a
  chromosome pair
Homozygous
 Having the same allele at the same locus on both members
  of a chromosome pair
Genotype
 The genetic makeup of an individual; the alleles at a
  particular locus
Phenotype
 The observable or detectable physical characteristics of an
  organism
            Terms used in genetics
 Recessive traits are not expressed in
  heterozygotes
   – For a recessive allele to be expressed, there
     must be two copies of the allele
 Dominant traits are governed by an allele that can
  be expressed in the presence of another, different
  allele
   – Dominant alleles prevent the expression of
     recessive alleles in heterozygotes
         The Principle of Segregation
 Genes occur in pairs because chromosomes occur
  in pairs
 During gamete production, members of each gene
  pair separate so each gamete contains one
  member of each pair
 During fertilization, the full number of chromosomes
  is restored and members of a gene or allele pairs
  are reunited
                The Principle of
            Independent Assortment
 The distribution of one pair of alleles into gametes
  does not influence the distribution of another pair
 The genes controlling different traits are inherited
  independently of one another
 Random assortment
   – The chance distribution of chromosomes to
     daughter cells during meiosis
   – A source of variation
             Patterns of Inheritance
 Trace inheritance of traits through generations of
  offspring
Autosomal traits→ dominant/recessive
X-linked traits→ dominant/recessive
 Assign a symbol for each allele
Dominant allele → capital letter
Recessive allele → lower case letter
Punnett Square
        The Punnett square
         demonstrates the probable
         combinations of a genetic
         cross
        Demonstrates how alleles can
         be combined when the F1
         plants are self-fertilized to
         produce an F2 generation.
        Shows that 1/4 of the F2
         plants should be homozygous
         dominant (TT), 1/2
         heterozygous (Tt), and the
         remaining 1/4 to be
         homozygous recessive (tt).
                   Mendelian Inheritance
 Tongue rolling: this involves        Assign a letter to represent the
  the ability to roll your tongue               alleles: R and r
  longitudinally into a U-shape.            R = dominant allele
  Many people are incapable of
                                            r = recessive allele
  performing this feat. Rollers
  may need to practice a few          One gene, two alleles produces
  times to do this, but nonrollers      three genotypic combinations
  will never be able to do it. The   RR homozygous dominant
  ability to roll is inherited as    Rr     heterozygous
  dominant, so those who             rr     homozygous recessive
  cannot roll their tongues are
  homozygous recessive.
Answer the questions regarding tongue rolling. Use
  the following symbols R (dominant allele) and r
  (recessive allele).
 If two heterozygous individuals reproduce, what
  percentage (or ratio) of the offspring would be able
  to roll their tongue? What percentage would not be
  able to roll their tongue?
Monogenic trait (Mendelian        Polygenic trait
  trait)                           A trait that is influenced by
 A trait that is influenced by     two or more loci →
  one genetic locus → one           multiple genes, multiple
  gene two alleles                  alleles
           Examples:                         Examples:
         Tongue rolling                      Skin color
            Earlobes                          Stature
Continuous Expression of a Polygenic Trait




 Represents the continuous expression of height in a large
  group of people.
                  Codominance
 The expression of two alleles in heterozygotes.
 In this situation, neither allele is dominant or
  recessive; thus, both influence the phenotype.
                   The ABO System
 The ABO system is governed by three alleles A, B, and O,
  found at the ABO locus on the ninth chromosome. These
  alleles (IA, IB, i) determine which ABO blood type an
  individual has by coding for the production of special
  substances called antigens. (Antigens are molecules at the
  surface your red blood cells that are “self” markers i.e., they
  identify the cells as being part of your own body). If only
  antigen A is present, the blood type (phenotype) is A; if only
  B is present, the blood type is B; if both are present, the
  blood type is AB; and when neither is present, the blood
  type is O.
    ABO Genotypes and Associated
           Phenotypes

             Antigens on      ABO Blood Type
Genotype
            Red Blood Cells    (Phenotype)

 AA, AO           A                 A

 BB, BO           B                 B

  AB            A and B            AB

  OO             None               O
 The ABO system illustrates dominance, recessiveness
  and codominance.
 The O allele is recessive to both A and B; therefore, if a
  person has type O blood, he or she must be homozygous
  for the O allele. However, since A and B are dominant to
  O, an individual with blood type A can actually have one of
  the two genotypes: AA or AO. The same is true of type B,
  which results from the genotypes BB and BO.
 Codominance is seen when two different alleles occur in
  heterozygous conditions where both alleles are expressed,
  and thus influence the phenotype. Therefore, when both A
  and B alleles are present, both A and B antigens can be
  detected on the surface of red blood cells.
    Answer the questions regarding the ABO blood
     system. Use the following symbols IA, IB, and i .
 A woman with type AB blood marries a man whose
  mother was type O and father type A. They have a
  son with type B blood. What are the possible
  genotypes of the father?
 If two people who have blood type A (both with the
  AO genotype) have children, what proportion of
  their children would be expected to have the O
  blood type? Why?
 Can the parents in question two have a child with
  AB blood? Why or why not?

				
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posted:7/26/2011
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